The control of nitrogen has become extremely important in view of its influence on chemical, mechanical and physical properties of steel and other metals. For example, nitrogen in steel causes poor deep drawability of thin sheets and low toughness of thick plates. Nitrogen content also affects the magnetic, electrical and welding properties of sheet steel. The hardness, abrasion-resistance and corrosion performance is also affected by the presence of nitrogen. Nitrogen may also be one of the causes of porosity because of gas evolution during solidification. In combination with aluminum, as aluminum nitride, it can be the direct cause of intergranular fracture of cast steel. Thus, nitrogen has been known as a detrimental element in many steels and other metals and its content should be reduced to a minimum. Conventional steel making processes are capable of reducing nitrogen content to less than 40 ppm. However such processes are generally incapable of reducing the nitrogen content in steel and other metals to less than about 20 ppm unless special expensive high quality starting materials are used.
In principle, two methods have been attempted to remove nitrogen from molten steel and other metals. They are vacuum degassing and flux treatment. Vacuum degassing was first tested to remove nitrogen from molten steel, since hydrogen in molten steel can be effectively removed by vacuum degassing to about 1 ppm. Unfortunately, however, vacuum degassing has been found to be an ineffective method for nitrogen removal, since only 10-30% of nitrogen in molten steel can be removed by vacuum degassing process. The significant difference in efficiency between nitrogen removal and hydrogen removal is attributed to nitrogen's smaller diffusion coefficient in molten steel. Therefore, research has focused on searching for slag systems that will effectively remove nitrogen from molten steel and other metals.
Extensive research was conducted for dozens of binary, ternary or multi-component slag systems containing CaO, BaO, MgO, Al2O3, SiO2, B2O3, CeO2, ZrO2, and TiO2 and many other oxides to discover effective slag systems for nitrogen removal from molten steel. Most of the studies attempted to measure nitride capacity as one of the indexes to express the ability of a slag to remove nitrogen. Despite progress made by previous studies, further efforts are required to find more effective slag systems.